Described below are a carbon dioxide sensor for determining the carbon dioxide content of air and a method for creating a measured gas value which represents the carbon dioxide concentration in air.
The detection of carbon dioxide is of great interest for a number of applications. Examples are the assessment of air quality in internal spaces, energy-efficient control of air conditioning systems or checking cleaned air. The aim of carbon dioxide detection can be to enhance comfort. It is however also possible to achieve significant energy savings under some circumstances.
Thus, for example, in a well insulated building, almost half the energy needed for air conditioning can by saved by demand-driven air conditioning. The demand is oriented inter alia in such systems to the carbon dioxide content of the air. In the automotive field too a demand-driven ventilation and air conditioning of the passenger compartment is advantageous. A figure for reduction in the fuel consumed for air conditioning is estimated at 0.3 l per 100 km.
Under normal environmental conditions carbon dioxide occurs in the air in a concentration or around 380-400 ppm. A sensor for carbon dioxide must be capable, using this basic concentration as its starting point, of detecting increased concentrations of up to 4000 ppm for example. It is problematic in such cases that the carbon dioxide molecule is a linear, symmetrical molecule and therefore no electrical dipole moment is present, which with various transducer principles can give rise to a sensor signal. Furthermore the molecule is chemically very unreactive.
At present very successful methods for determining the concentration of carbon dioxide are therefore to be found primarily in the area of optical spectroscopy. These methods make use of the fact that in specific wavelength ranges, for example at around 4.3 μm wavelength, carbon dioxide absorbs light. This makes possible an exact and selective measurement of the concentration of carbon dioxide. In such cases it is not a matter of the chemical reactivity of the carbon dioxide. The disadvantage of optical spectroscopy is however the complex structure of the measurement systems and the significant outlay necessary for evaluating the measured spectra. This ultimately leads to comparatively large and expensive measurement systems.
Solid-state sensors, such as semiconductor gas sensors for example, avoid the disadvantages of the optical measurement systems. They are small, extremely cheap to manufacture by comparison through mass production and need a less complex signal evaluation. However the disadvantage of solid-state sensors is that they are dependent on a certain reactivity of the molecules to be measured and simultaneously however detect all molecules which just have a certain reactivity. To put it another way, the solid-state sensors have a low selectivity. Above all this makes the measurement of less reactive species such as carbon dioxide difficult with such sensors, since they mostly react very strongly to hydrocarbons or ozone.
The number of potential interference gases is substantial in such cases. It includes nitrogen dioxide (NO2), carbon monoxide (CO) and hydrogen (H2), ammonia (NH3), ethanol or hydrochloric acid (HCl), nitrogen monoxide (NO), sulfur oxide (SOX), carbonyl sulfide (COS), laughing gas (N2O) and prussic acid (HCN), water (H2O) as well as organic gases such as methane, ethane, ethene, acetylene and other hydrocarbons such as formaldehyde (CH2O). Other interference gases are amines (NH2R1, NH1R2, NR3), amides (RC(O)NH2, RC(O)NHR′, RC(O)NR′R), acrolein (C3H4O) and phosgene (COCl2), aromatics such as benzol (C6H6), ethyl benzol, chlorbenzol, toluol, xylol, styrol and phenol (C6H6O). There is also ozone (O3), the large group of VOCs (volatile organic compounds).
Some of these gases already occur in normal ambient air, for example ozone. Further sources for gases are fires, cigarette smoke, human activity, the use of chemical media such as cleaning agents, foodstuffs left open or technical devices such as printers. Road traffic and even the weather conditions also lead to the occurrence of gases.
The publication by H.-E. Endres et al., “A capacitive CO2 sensor system with suppression of the humidity interference”, Sensors and Actuators Vol. 57 (1999), 83-87 discloses a CO2 sensor which is based on the principle of capacitance measurement. With the disclosed capacitive sensor an additional humidity sensor is used in order to generate a humidity signal.
A potential-controlled humidity sensor which is able to be used for this purpose is known for example from EP 1 176 418 A2. The potential-controlled humidity sensor has a gas-sensitive area which is able to be polarized independently of a humidity. Furthermore the gas sensitive area exhibits a relative dielectricity constant, which is dependent on the humidity.
The disadvantage of the known sensors is that a further sensor is necessary to determine and compensate for air humidity. Just like the actual carbon dioxide sensor, this must be calibrated and evaluated and must also fulfill the characteristics required for the carbon dioxide sensor, for example longevity.